2000 Ford Taurus purging coolant

My 2000 Taurus needs a coolant flush (rusty film in system) but I need to know if there may be another issue as coolant purges out the cap of the reservoir after the car has been running at temperature for a while. Eventually, it purges enough to cause overheating and more purging. The reservoir is an aftermarket unit with vented cap. Electric fans are functioning but not sure if they are switching on soon enough. Thank you.
Harley